Numerically controlled (N.C.) machine tools have been developed to an extraordinarily high level of technology and have become quite reliable tools. However, known N.C. machines are not sufficiently equipped for checking out or examination of whether the performance of the machine is properly worked out or not. To the extent that appliances for such checking or examination have been available, they have been complicated in structure and expensive.
The assignee of the present invention once offered one sensing system (here shown in FIGS. 1, 2 and 3) for drilling machines, including a revolving cutting tool 3, a cutting tool supporting block 4, a screw thread spindle 6 and a ball thread appliance A. In that prior sensing system, the feed rate of the cutting tool is represented by the indicator of the meter B, which indicates when the real feed rate is faster or slower, and the feed rate of the tool is controlled by observing movement of the indicator of the meter. The said meter makes a comparison of two physico-electric quantities M1 and M2. The quantity M1 represents real feed rate per unit of time and is an electric potential quantity (voltage) generated in such a way that linear cutting motion of the tool supporting block 4, which drives cutting tools such as drills, taps and other kinds of cutting tools and advances and retracts the tool with respect to the workpiece, is converted to revolving motion of the screw thread spindle 6 by means of the ball-thread appliance A. Said screw spindle 6 drives a generator 8 (FIG. 3), by which an electric potential is generated and is multiplied by a number l representing the lead length of said screw spindle. The other quantity M2, representing adapted (desired or preselected) cutting feed rate per unit time, is a quantity obtained by multiplying a generated electric potential by a number L representing the adapted cutting feed (or a predetermined pitch length per revolution). The latter electric potential is generated by a generator 12 driven by a revolving spindle which revolves at the same rotational speed as the cutting tool spindle 9.
Such equipment uses generators. The frequency of the generated current becomes low and rectification does not occur properly because the cutting feed rate becomes too low. Accordingly, the indicator of the meter does not stabilize.
Also, the electric potential output of generator G1 cannot linearly follow abrupt changes in the feed motion, causing the indicator of the meter to lag behind sudden changes in feed motion. For example, if the feed rate is changed, as from rapid feed to cutting feed, the indicator does not return rapidly because the generator voltage is high in rapid feed. This high voltage must be discharged when the feed is changed to the lower, cutting feed rate, and this discharge requires some time, thus delaying the return of the indicator to its initial point.
Also, if the tool motion stops during cutting, the potentials M1 and M2 each become zero. Thus, if a comparison thereof is made by dividing M1 by M2, the resulting calculation is written 0 volts/0 volts and the location of the indicator of meter B is indeterminate. Such are the defects of the previous device.
An object of this invention is to offer a new cutting feed sensing device for a drilling machine, which avoids the above-mentioned defects of the prior sensing device.